1. Field of the Invention
The present invention relates to polystyrene beads and to products made using those beads. More specifically, the invention relates to both unexpanded and expanded polystyrene beads with specific shapes and dimensions that make the beads perform well in particular applications.
2. Description of the Prior Art
Polystyrene beads are well known, and have been used extensively for some time. These beads start out in an unexpanded state, and can be expanded, using known processes such as those disclosed in U.S. Pat. Nos. 5,573,790 and 5,267,845, into expanded polystyrene (EPS) beads. During this process, the beads can be expanded to different extents. The density of the resulting EPS beads can be measured in units of pounds per cubic foot.
Expanded polystyrene beads have previously been available in a number of different shapes and sizes, and used in a variety of applications. For example, small spherical EPS beads have been molded together to form solid polystyrene foam structures. Common sizes for these spherical beads are, for example, 0.5 mm to 1.5 mm. Larger EPS beads have also been used as packing material, or "peanuts", to protect fragile items during shipping. These "peanuts" are known to have, for example, S, H, I, and C shapes, and sizes on the order of 2 to 4 cm. Cylindrical EPS beads measuring 7 mm in diameter by 15 mm in length were used briefly in the 1980s by ceiling texture manufacturers, who shredded the beads and used the particles as filler. These beads were expanded from resin beads measuring about 2 mm in diameter by 5 mm in length.
Until now, however, attempts to use EPS beads as an ingredient in concrete mixes have suffered from serious drawbacks.
Concrete is made by mixing together cement, water and aggregate. Traditionally, the aggregate included a mixture of fine mineral aggregate (e.g., sand) and coarse mineral aggregate (e.g., gravel). One commonly used concrete mix uses 1 cubic foot of cement, 2 cubic feet of sand, 3 cubic feet of gravel, and 51/2 gallons of water. When these ingredients are mixed together, the cement begins to set, and it eventually hardens into a strong, solid material.
One drawback of concrete made from these ingredients is that it is very heavy, due in large part to the weight of the aggregate. In certain situations, particularly when the strength of the concrete is not a limiting factor, a lighter concrete would be advantageous.
In the past, attempts were made to make a lightweight concrete by replacing some of the sand and gravel with spherical EPS beads. But these previous attempts failed because the beads tended to agglomerate (i.e., clump together) in the concrete mix, which made it difficult to make uniform pieces of hardened concrete. In addition, the spherical shape of the EPS beads did not bond well to the concrete, which weakened the resulting concrete to an unacceptable degree.